1. Field of the Invention
The present invention relates to a process of producing a neodymium-iron alloy and an apparatus for producing the same. More particularly it relates to a process of continuously producing or manufacturing a neodymium-iron alloy of high neodymium content, which can be advantageously used as a material for a high-quality permanent magnet and free from containing, for that use, harmful impurities and non-metallic inclusions.
2. Description of the Prior Art
Recently high-quality permanent magnets, made of rare earth and iron or rare earth, iron and boron, which do not contain expensive samarium and cobalt and which are also superior in the magnetic properties to the hard ferrite, have been drawing public attention. Above all, a permanent magnet consisting of neodymium, iron, and boron is generally recognized as an excellent material for a maximum energy product, (BH) max more than 36 MGOe, and also for its superiority in its weight-to-volume ratio and mechanical strength (a Japanese laid open patent application: TOKU-KAI-SHO-59 (1984)-46008 can be referred). In this type of permanent magnet, made of neodymium and iron or neodymium, iron, and boron, it is essentially required to obtain a material or materials containing least possible impurities which deteriorate magnetic properties, and to industrially establish a manufacturing process, particularly as to a neodymium material which is high in reactivity, of getting one containing a minimum of impurities, for example, oxygen, as possible.
Metallic neodymium has been, in fact, regarded almost useless, and the industrial manufacturing process of obtaining the same has not been settled, yet, except only for the method of reducing a neodymium compound by utilizing an active metal, especially calcium, and for that of electrolyzing the same in an electrowinning bath, i.e., a fused salt electrolyte. It can therefore be said that no industrial process is firmly established for producing a neodymium-iron alloy which is suitable for being used as a permanent magnet of the type mentioned above.
Processes, which can be named at the present level of the technology, of manufacturing the neodymium-iron alloy, under those circumstances, are described below. All of them, however, are not satisfactory, because of inherent disadvantages or problems, and practical limitations for containing industrial processes.
(a) A method wherein metallic neodymium is prepared beforehand by means of reducing a neodymium compound with an active metal such as calcium or by means of electrowinning the same in a bath of electrolyte, and the obtained metallic neodymium is melted together with iron for alloying them:
The method, however, is problematical in the first step of preparing the neodymium metal. The reduction method utilizing an active metal such as calcium belongs to a batch system, so to speak, which is not suited for a continuous operation in a large scale. In the electrowinning method, two techniques can be named as a prior art: Electrolysis in an electrolyte bath of fused chlorides (see Jiro Shiokawa et al. in "Denki Kagaku (Electrochemistry)" Vol. 35, pages 496 et seq. (1967), and others) and electrolysis of oxide (Nd.sub.2 O.sub.3) dissolved in an electrolyte bath of fused fluorides (see E. Morrice et al., "U.S. Bur. of Min., Rep. of Invest."., No. 6957, 1967). All of them can not be an established method suitable for a continuous and large scale operation, still containing some defects and problems in their results of electrolysis and methods of operation.
(b) Another method wherein alloying is executed by means of reducing a mixture of a neodymium compound and an iron compound or iron by utilizing a reducing agent such as calcium:
This method can not be, either, an alternative the general reduction method carried out in a batch style, and is unsuitable for a continuous and large scale operation.
(c) Still another method wherein an alloy of neodymium and iron is deposited on so-called unconsumable cathode by simultaneous electrolytic reduction which is carried out in a bath of electrolyte dissolving both a neodymium compound and an iron compound therein:
This method is economically inferior even to the undermentioned method (d), because the composition of the alloy can not be kept constant or uniform, and the iron obtained is too expensive. Iron is obtainable in a large scale and less expensive in an ordinary method, not by this uneconomical process using the electrolysis of the fused salts.
(d) The so-called consumable cathode method, wherein the process of depositing the metallic neodymium on a consumable cathode of iron and the alloying process between the neodymium and the iron simultaneously occur in one electrolytic reduction step of the neodymium oxide (Nd.sub.2 O.sub.3) as a neodymium compound, executed in a suitable bath of an electrolyte of fused salts.
As to this method an experimental study is disclosed by E. Morrice et al. in a publication of "U.S. Bur. of Min., Rep. of Invest.", No. 7146, 1968. This method, wherein electrolysis is executed in a bath of electrolyte of fused fluorides by adding neodymium oxide thereinto, is considered far superior to the above-introduced three methods, from (a) to (c), not being subject to faults inevitable to those prior art method. The method, however, is still not free from some inherent shortcomings from a technological viewpoint.
The shortcomings will be described in more detail: the solubility of the neodymium oxide in the selected electrolyte bath is as low as 2% in this method which uses the neodymium oxide as its raw material; moreover, the solubility tends to become lower, because the temperature of the electrolyte bath must be selected to be as low as practical for the purpose of obtaining an alloy with as little impurities as possible as stated in the object of the present invention, and the lower temperature of the bath makes the dissolution of the neodymium oxide more difficult. As a consequence, difficulty of continuous and stable supplying of the raw material to the bath will cause the undermentioned problems, which hinder the industrial application of this process where the continuous operation is essential.
(1) An abnormal phenomenon called "anode effect" occurs frequently due to shortage of the raw material dissolved in the electrolyte bath. The anode effect is well known to be specific to the electrolysis of the fused salts, particularly fluorides. (2) The undissolved raw material prevents liquid drops of the produced alloy from coalescing. (3) The undissolved raw material tends to be precipitated on the bottom of the electrolytic cell as sludge. The sludge subsequently degrades the formed alloy due to inclusion of undesirable foreign matter, deteriorates the utilization yield of the raw material, and disturbs the electrolysis operation. (4) Too much occurrence of the anode effect deteriorates the electrolysis results. And (5) the continuation of the electrolysis itself encounters sometimes difficulties of various sorts.